Development of road grade data using the United States geological survey digital elevation model

•The USGS DEM is a valuable resource for generating road grade in the United States.•Spline smoothing is useful to tradeoff goodness-of-fit and smoothness of grade.•A larger weight on smoothness is required to estimate grade for highways.•A larger weight on goodness-of-fit is required to estimate grade for local roads.•Optimal smoothing parameter can be chosen based on elevation fluctuations.